Drive assembly for magnetic tape recorder

ABSTRACT

In a drive assembly for magnetic tape recorder a separate motor is associated with each of the reels. The motors are reversible so that the tape can be driven bidirectionally. An automatic control system maintains the constancy of the tape motion at any speed by monitoring the actual tape speed and comparing it with the nominal value thereof. Upon detection of a difference between the two data a servo loop is actuated to correct the deviation of the actual from the nominal speed. Adequate tension of the tape is maintained at all times.

- United States Patent- [72] Inventor Hubert Richt [56] References Citedswung Germ"! UNlTED STATES PATENTS P 839d 3,045,937 7/1962 Johnson242/203 PM My 3 104 071 9/1963 N b 242 ew erg /203 [451 3 141 626 7/1964Hoskin 242/203 [73] Assignec Suddeutsche Mechanische Werkstatten WolfFreiherr von Hornstein KG Primary Examiner-Leonard D. ChristianBarmseestr. ll, Munich, Germany Attorney-Nolte and Nolte ABSTRACT: In adrive assembly for magnetic tape recorder a [54] DRIVE ASSEMBLY FORMAGNETIC TAPE separate motor is associated with each of the reels. Themo- RECORDER tors are reversible so that the tape can be drivengchjmszmawmg Figs bidirectionally. An automatic control system maintainsthe [52] US. Cl 242/203, constancy of the tape motion at any speed bymonitoring the 242/208. 318/7 actual tape speed and comparing it withthe nominal value [51] Int. Cl 81 lb [5/32, thereof. Upon detection of adifference between the two data G03 b 1/04 a servo loop is actuated tocorrect the deviation of the actual [50] Field of Search 242/201, fromthe nominal speed. Adequate tension of the tape is maintained at alltimes.

PATENTED JUN22 |97| wsxr P/a/T INVENTOR ATTORNEYS DRIVE ASSEMBLY FORMAGNETIC TAPE RECORDER BACKGROUND OF THE INVENTION In prior driveassemblies for magnetic tape'recorders comprising a single motor, thefly wheels and also the capstan, are driven through one gearing, and thecoupling of the takeup reel is driven through another gearing or throughthe fly wheels, The tape is advanced by the capstan rotating at constantangular speed and is wound upon a spool supported by the takeupcoupling. For high-speed winding, respectively unwinding of the tape oneof reel couplings is driven at increased velocity while the other one isbraked. Simultaneously, the friction contact between the capstan and thetape is suspended by releasing the free-running pressure roller from thetape.

An assembly of this type requires a large number of mechanicalstructural elements for the various tape runs, for example, for standardspeed, for high speed and for the stop command. The high velocity ofcomponents such as the driving motor and the friction pulleys generatevibrations and background noise. Another drawback is the need for rubberdriving belts and rubber contact surfaces for the friction pulleys whichare subject to deterioration by dirt or wear, and to deformation.Furthermore, the sliding release couplings are liable to slip duringoperation of the assembly, while also causes interfering noise anddeterioration of the friction surfaces. In an assembly in which the tapeis driven by a capstan cooperating with a pressure roller it isimpossible to avoid a certain amount of tape slippage relative to thecapstan. The slippage is nonlinear because of inhomogeneities in therubber pressure roller, which are introduced during the manufacturingprocess, and results in fluctuations of the sound pitch and in drift ofthe tape speed. A further disadvantages of singlemotor assemblies isthatit is necessary, for remote control, to actuate not only the gearing andthe brakes for the reel couplings but the pressure roller, so thatadditional expensive electromagnetic actuating elements, such as forexample, solenoids, must be provided.

Another conventional drive mechanism comprises a separate motor for eachof the reels, as well as a driving motor rotating at constant speed. Theshaft of the motor forms the capstan, so that friction gearing, pulleysand driving belts are eliminated. Remote control of a three-motorassembly is simpler than control of a single-motor system because of thereduction in the number of structural mechanical elements. The tape canbe driven bidirectionally by reversing the rotational direction of themotors. However, since the driving motor has a high rotational energy,there is a substantial dead time after reversing the direction ofrotation, until the desired speed in the opposite direction is attained.

Although three-motor assemblies, with a lesser number of mechanicalstructural elements, are more reliable in operation than single-motorassemblies, they are considerably more expensive because they requirehigh quality and costly motors. The demands on the driving motors areparticularly strict with regard to quietness and steadiness ofoperation. The other components, for example, the capstan and thepressure roller must be high-precision parts. Since the tape isadvanced, as in single-motor drives, between the capstan and thepressure roller, a certain amount of tape slippage is unavoidable. Therisk of slippage is even greater in a three-motor system because thedriving motor is operated at relatively high speed, so that a capstanofsmall diameter is required to drive the tape at the proper speed. Asin singlemotor systems with remote controls, means must be provided toactuate the pressure roller electromagnetically which adds to theexpenses for the assembly.

In yet another known mechanism the drive capstan for the tape isdispensed with, and a DC motor is provided for the takeup reel couplingonly. One end of the motor shaft is frictionally engaged with a frictionsurface provided at the circumference of the lower part of the reelcoupling. The rotational speed of the motor is regulated by a DCgenerator whose rotor is driven by the tape, so that the tape speed iskept constant. The advantage of the mechanism which has neither acapstan nor fly wheels or a pressure roller, is offset by the noiseproduced by the high-speed driving motor. The vibrations of the motorand of the contact surface on the lower part of the reel coupling, whichis made of rubber, and the abrasion, contamination and deformation ofthis surface all affect the synchronization of operations in a way whichcannot be compensated for. If the motor shaft remains in contact withthe surface of the reel coupling when the mechanism is stopped,deformations such as dents are likely to be produced therein, whichultimately affect the symmetry of the coupling. A similar skewedalignment may result from the deposition of dust or abrasion particleson the contact surfaces.

Another drawback is that the motor must be pivotably mounted so that theshaft can be rapidly and selectively thrown into gear with either of thetwo reel couplings for highspeed rewind operations.

SUMMARY OF THE INVENTION It is an object of the present invention toovercome the disadvantages of known drive assemblies for magnetic taperecorders by providing a simple and economical assembly adapted forbidirectional operation.

Another object of the present application is to provide an improvedassembly substantially without mechanical structural elements andhigh-precision components.

A further object of the present invention is to provide an assemblywhich is reliable in operation, responsive to commands, and adapted tomaintain constant velocities.

These objects and others which will become apparent hereinafter areattained, in accordance with the present invention, through a pair ofreel motors coaxial with the spool supports, respectively the couplingsof each of the two reels. Preferably, motors of the deep-bar, squirrelcage motor type may be used. During operation of the assembly, therotational speed of one of the motors is regulated by a servo loop sothat the speed of the tape is kept constant for at least one selectedtape velocity. The actual speed of the tape is read off, as is wellknown, from markings provided on a roller which is turned by the motionof the tape, and which may be scanned inductively, capacitively,galvanically or optically. For example, if the markings are serrationsmade in a ferromagnetic surface and scanned by a polarized magnetichead, the frequency of the alternating current derived therefrom isproportional to the actual tape velocity. The frequency is compared witha reference frequency representing the nominal value. A differentialbetween the two valves which is equivalent to the instant deviation ofthe tape velocity from the desired value is fed as a signal to avariable gain amplifier which excites the driving motors for example,through an intermediate element. If the actual tape velocity is toohigh, or too low, the amplifier causes driving current to be fed to thewindings of the respective motor proportional to the correction requiredto counteract the deviation. Simultaneously, the opposite motor isenergized to drive at a speed and in a direction which maintainsadequate tape tension.

It is also possible to regulate the tape speed by controlling forexample, the speed of the payoff reel motor by the means described abovewhile the motor of the takeup reel is driven without controls.

When the desired velocity of the tape is to be changed for recording orreproduction, a different nominal value corresponding to each of thespeeds is used as a reference, so that the tape speed can be keptconstant by comparison therewith.

The driving motors for the reels, according to a preferred embodiment ofthe above invention, are of the type having a constant torque which isindependent of the position of the rotor and sufficient to overcome theinertia of the spool supports, respectively of the reel couplings.Suitable motors are, for example, DC or induction motors with anadequate number of poles, or DC or induction motors whose velocity,

respectively torque, can be conveniently regulated. Of particularadvantage are deep-bar squirrel cage rotor-type motors whose torque isindependent of the rotor position and which have a relatively hightorque to low inertia ratio.

The use of two-reel motors driven at low speeds eliminates, to a verylarge extent, motor vibrations which affect the desired steadiness ofthe tape run. lrregularities in the motor drives and the sliding noiseof the reel couplings are also avoided. The rotational energy of thedrive mechanism components is relatively low, so that a fast switchingto another tape velocity, or reversal of the tape drive direction, doesnot subject the tape to a large tension impulse.

According to a preferred embodiment of the invention the reel couplingsare mounted on the rotors of the reel motors, so that jerky impacts onthe tape during a reversal of direction, or following the stop command,can be avoided. The couplings have the dual purpose of acting asarresting brakes.

In a preferred embodiment the constancy of the tape speed is maintainedeven for high-speed runs, which makes it easy to locate adesired-position on the tape. A constant coefficient coordinates thehigh-speed run with each of the standard tape velocities, so thathigh-speed operation requires a constant fraction of the recording time,independent of the tape velocity during the recording operation. This isparticularly advantageous for finding a particular place on the tapesince the temporal relation of individual recording positions ispreserved.

The drive assembly, according to the present invention, comprises asingle servosystem for both reversible motors to control the tape speed,and a single voltage source for producing opposite torques which can beswitched over when the direction of the tape transport is to be changed.I

The coaxial arrangement of the motors and i the real couplings makes itpossible to drive the tape in both directions. The change from onedirection to the other can be achieved simply and economically byswitching the control system for the regulation of the tape speed andthe voltage source for the production of the torque, from one motor tothe other. In either direction of motion of the tape the standardvelocities and high-speed operation can be regulated through feedback ofthe actual values and comparison with the nominal values correspondingto the desired speeds.

A further characteristic of the invention is that an additional markingis provided on the roller driven by the tape. The marking which may bescanned by inductive, capacitive galvanic or optical detectioninstruments serves to measure the tape length. According to a preferredembodiment of the invention, the marking is a line which is scannedphotoelectrically and transmitted as a signal to a suitable counter.

The roller, which is partially encircled by the tape, moves easily andis adapted to steady the tape by damping the longitudinal oscillationsthereof. According to a preferred em bodiment the roller is providedwith tape guides, so that additional guides, for example, guide bolts,are not required.

The distinctive features of the present invention lie in the almostcomplete absence of mechanical structural elements, of high precisionparts, of special gearing, particularly of gearing including rubbercomponents such as friction pulleys or belts, and of high-speedelements. In operation, the assembly is extremely quiet, reliable andeconomical. It responds fully to remote controls and can be equivalentlyoperated in both directions.

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary frontelevational view of a drive assembly according to the present invention;and

FIG. 2 is a somewhat diagrammatic plan view of the drive assembly shownin FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 there is shown a pairof motors 1 and 2 of the external rotor type, preferably of the deep-barsquirrel cage rotor type. The mounting of such motors in the frame of amagnetic tape recorder is well known to those skilled in the art andrequires no further explanation. A pair of couplings 5 and 6 connect therotors 3 and 4 of the motors l and 2 with a pair of spool supports 22and 23 for the reels 7 and 8 which are supported thereon in coaxialalignment with the motors.

At any one time one of the reels 7 and 8 serves as the payout reel andthe other one as the takeup reel. A tape 9 extends between the reels 7and 8 and is adapted to be wound upon the takeup reel and simultaneouslyunwound from the payout reel upon energization of the respective motors.The path of the tape 9 between the reels 7 and 8 leads past the erasingheads L and L as well as past a magnetic head assembly K therebetweenwhich is coupled to recording and reproducing circuits (not shown). Onits path between the reels 7 and 8, the tape 9 also passes over rollers10 and 11, the latter being rotatably attached to the frame or housingof the instrument and symmetrically positioned intermediate the reels.The rollers l0 and 11 are guides for the tape 9 and are rotated when thetape is moved over and engages them, so that their angular velocity isproportional to the linear velocity of the tape. Markings 19 areprovided on the circumference of the roller 11 from which the velocityof the tape motion can be obtained by suitable scanning means, such asM. Markings 21 shows as line markings applied to the circumference ofthe roller 10 are adapted to be read by a photoelectric scanner P todetermine the length of the tape transported between the reels 7 and 8,respectively wound upon one of these reels.

In the illustration of FIG. 1 the drive assembly is in the Go position,wherein the locking elements 12 and 13 which are pivotally mounted at 24and 25 inthe frame of the instrument, are withdrawn from engagement withthe rotors 3 and 4 of the motors l and 2. The electromagnetic release,of the elements 12 and 13 from the rotors 3 and 4 is actuated by closingthe switch S (FIG. 2), thus causing current to flow from a power source15 to the solenoid l4. Excitation of the solenoid 14 attracts thearmature 16 against the force of spring 17 to which it is attached. Theelements 12 and 13 which are symmetrically and pivotally attached at oneof their ends to the armature 16, are displaced from their lockingposition when the armature is drawn toward the interior of the solenoid14, thus releasing the rotors 3 and 4. If the excitation of the solenoid14 is interrupted, for example upon the stoP" command, or upondisconnection of the drive assembly and switching off of the motors 1and 2, the armature 16 is pulled out of the solenoid 14 by the force ofthe spring means 17, so that the locking elements 12 and 13 are againmoved toward the rotors 3 and 4. In this way the rotor of the motorassociated with the payout reel is firmly held by one of the lockingelements 12 and 13 whereas the rotor of .the motor associated with thetakeup reel is only lightly braked. For example, if the tape 9 istransported in the direction of the arrow 18 the rotor 3 is instantlylocked by the locking element 12 whereas the rotor 4 is only lightlybraked. The tension of the tape 9 is regulated essentially by thelocking momentum of the coupling on the payout reel, that is, in thepresent example by the coupling 5, so that slack loops of tape areprevented.

The speed of the tape 9 is controlled by comparing the actual speed withthe nominal speed assigned to the respective tape drive velocity and bycorrecting any deviation therefrom. For this purpose the actual speed iscontinuously checked by scanning the markings 19 applied to thecircumference of the roller 11. The markings 19 which may be serrationsmade from ferromagnetic material pass in front of a scanner such as apolarized magnetic head M which is connected to a suitable electriccircuit. As the roller 11 is rotated by the motion of the tape 9 trainedover it, a current is produced in the circuit whose frequency isproportional to the instant speed of the tape. Fed as a signal into thecomparison element V, the respective frequency is matched against anominal value 0 stored therein. Any difference between the actual valueand the nominal value is transmitted as another signal to a variablegain amplifier R which regulates the speed of the motor 2 through anappropriate correcting element (not shown). If the instant tape speed isslower than the nominal value, the described automatic control systemcauses motor 2 to be excited until the desired tape speed is produced.Corresponding corrections are made if the instant tape speed is toofast. If the speed of the tape 9 is too slow, that is, if motor 2 istemporarily driven with increased excitation, a voltage lower than thenominal voltage is fed from the source Sp to the motor 1 to produce acounter torque which is adequate to maintain tension in the tape for thewinding process and for contact with the heads L,, K and L The actualtape speed readout from the markings 19 on the roller 11 at differentstandard type velocities and during highspeed winding, respectivelyrewinding operation, can be matched against nominal values correspondingto the respective velocities and stored in the appropriate memorypositions of the servoloop. in this way, the automatic control systemcan immediately correct any deviation of the actual speed from thenominal speed for different tape velocities.

In a likewise manner it is possible to switch the motor 1 on the payoutreel 7 to automatic control for maintaining a constant speed, whilemotor 2 which drives the take up reel 8 is supplied directly with therated voltage from the source Sp. The shifting from one system to theother is easily accomplished by disconnecting the terminals e-f and b-cin the flow diagram and by connecting the terminals d--f and ac instead.

The assembly, including the tape, is equivalently operable in eitherdirection, that is in the direction indicated by the arrow 18 or in thedirection'of the arrow 20. in order to drive the tape at controlledspeed in the direction of arrow 20 it is only necessary to switch themotor 2 which is, in this case, coordinated with the payout reel toautomatic control and supply the motor 1 coordinated with the takeupreel with thenominal voltage from the source Sp. Alternately the takeupreel motor 1 may be switched to automatic control, and the payout reelmotor 2 may besupplied with less than the rated voltage from the sourceSp to develop a counter torque so that adequate tension of the tape 9can be maintained. For the latter arrangement it is necessary to connectthe terminals ac and df.

Through the above-described automatic control means it is possible tokeep the tape speed constant in either direction of transport and at allstandard velocities as well as at high-speed operation.

What I claim is:

l. A drive assembly for a magnetic tape recorder comprising a pair ofmotors each having a drive shaft, a power source for driving saidmotors, means for selectively driving said drive shaft at differentspeeds, means for regulating the speed of either of said drive shafts,reversing switch means for changing the direction of motion of saiddrive shafts, a reel operatively connected with each of said driveshafts and rotatable therewith, a tape wound on and extending betweensaid reels, means for winding said tape upon one of said reels at apredetermined speed and simultaneously unwinding said tape from theother of said reels, means for measuring the actual speed of said tapeadvanced by said reels, means for converting said measurements intosignals, means for correlating said signals with stored signalsrepresentative of the nominal speed corresponding to at least one ofsaid predetermined tape speeds, and an automatic control system forcorrecting drift of the actual tape speed relative to the nominal tapespeed whereby constancy of motion of said tape is obtained.

2. A drive assembly as defined in claim 1 wherein said motors areexternal rotor-type motors.

3. A drive assembly as defined in 'claim2 wherein said motors aredeep-bar squirrel cage rotor-type motors.

4. A drive assembly as defined in claim 2 comprising a spool support foreach of said reels and a coupling coaxially mounted on each of saidmotor rotors connecting said motors and said spool supports.

5. A drive assembly as defined in claim 1 wherein said speed measuringmeans comprise at least one roller rotating at an angular speedproportional to the linear speed of said tape, at least one markingapplied to the circumference of said roller, and a counter registeringthe number of revolutionsof said roller per unit of time.

6. A drive assembly as defined in claim 5 wherein said marking is aserration formed from ferromagnetic material and said registeringcounter comprises a polarized magnet proximate to said roller havingsaid marking.

7. A drive assembly as defined in claim 5 wherein said marking is a lineand said registering counter comprises a photoelectric cell.

8. A drive assembly as defined in claim 7 comprising means forcorrelating the number of revolutions of said roller with a numberrepresentative of the length of the tape.

9. A drive assembly as defined in claim 1 comprising a single automaticcontrol system for regulating the speed of said motorin either directionof motion, a single voltage source for producing a counter torque ineither of said motors, means connecting said automatic control systemand said voltage source with either of said motors, and means forswitching said automatic control system and said voltage source from oneof said motors to the other of said motors during a reversal of thedirection of motion of said motors.

1. A drive assembly for a magnetic tape recorder comprising a pair ofmotors each having a drive shaft, a power source for driving saidmotors, means for selectively driving said drive shaft at differentspeeds, means for regulating the speed of either of said drive shafts,reversing switch means for changing the direction of motion of saiddrive shafts, a reel operatively connected with each of said driveshafts and rotatable therewith, a tape wound on and extending betweensaid reels, means for winding said tape upon one of said reels at apredetermined speed and simultaneously unwinding said tape from theother of said reels, means for measuring the actual speed of said tapeadvanced by said reels, means for converting said measurements intosignals, means for correlating said signals with stored signalsrepresentative of the nominal speed corresponding to at least one ofsaid predetermined tape speeds, and an automatic control system forcorrecting drift of the actual tape speed relative to the nominal tapespeed whereby constancy of motion of said tape is obtained.
 2. A driveassembly as defined in claim 1 wherein said motors are externalrotor-type motors.
 3. A drive assembly as defined in claim 2 whereinsaid motors are deep-bar squirrel cage rotor-type motors.
 4. A driveassembly as defined in claim 2 comprising a spool support for each ofsaid reels and a coupling coaxially mounted on each of said motor rotorsconnecting said motors and said spool supports.
 5. A drive assembly asdefined in claim 1 wherein said speed measuring means comprise at leastone roller rotating at an angular speed proportional to the linear speedof said tape, at least one marking applied to the circumference of saidroller, and a counter registering the number of revolutions of saidroller per unit of time.
 6. A drive assembly as defined in claim 5wherein said marking is a serration formed from ferromagnetic materialand said registering counter comprises a polarized magnet proximate tosaid roller having said marking.
 7. A drive assembly as defined in claim5 wherein said marking is a line and said registering counter comprisesa photoelectric cell.
 8. A drive assembly as defined in claim 7comprising means for correlating the number of revolutions of saidroller with a number representative of the length of the tape.
 9. Adrive assembly as defined in claim 1 comprising a single automaticcontrol system for regulating the speed of said motor in eitherdirection of motion, a single voltage source for producing a countertorque in either of said motors, means connecting said automatic controlsystem and said voltage source with either of said motors, and means forswitching said automatic control system and said voltage source from oneof said motors to the other of said motors during a reversal of thedirection of motion of said motors.